This invention relates generally to frequency correction apparatus and, more particularly, to frequency correction apparatus adapted to correct the frequency error in a reference signal produced in an oscillator and caused by variations in the environment of the oscillator.
As is known in the art, the frequency of a reference signal may vary as a function of the operating environment, such as the temperature of the source of such signal. Correction of such variations in the frequency of the signal source is usually achieved by either analog or hybrid digital-analog techniques. Analog techniques include the product modulation of the desired frequency modulation spectrum on the source signal. Hybrid techniques often employ a phase-locked loop with an inverted phase version of the desired modulation added to the phase detector output thereby tracking the desired modulation into the VCO frequency. Either technique has great difficulty with the requirement of extremely accurate modulation, particularly at extremely low modulation frequencies.